I know of a few people who code in Django, and there is a book coming out soon that is in “Beta” online, and I have to say it is the most intuitive collaborative book editing system I have seen. Great idea, excellent execution.
Anybody can add a comment to the sidebar, and they are indicated by a little icon (seen in my screenshot). I think that someone could recycle this idea for blogs. Imagine if instead of having comments be at the end of the post, they could be inline? I think that would rock.